Membrane filters of graded permeability are produced according to various methods and from various materials. They consist, for example, of a thin film derived from colloidal solutions, such as gelatin, collodium, silicic acid, cellulose esters or cellulose, which, upon evaporating and coagulating, form a lattice work the cavities of which form a sponge-like structure. Depending upon the production method, the cross-sections of the pores may vary greatly, so that for example bacteria and even particles up to molecular size may be screened off.
In general, membrane filters have to be supported in use; larger ones mostly on sieve-plates or frits, because by themselves they are not sufficiently strong to withstand mechanical stress.
For many purposes a solid composite material made of membrane filter and a porous backing would be very desirable, which completely preserves the individual properties of the membrane filter on the one hand and the properties of the porous backing on the other hand.
It is known to use an adhesive for pasting the membrane filter on a porous backing, but this method has the disadvantage that the adhesive may influence in an unpredictable manner the properties of the system membrane filter/porous backing. It is also known to impregnate a porous backing with a solution which forms a membrane filter upon drying, so that the membrane layer forms at least partially within the porous material. But the product thus formed has properties other than those of the individual components. This defect applies especially to the surface, which no longer exhibits the smooth membrane filter surface required for many types of work, but takes on the structure of the porous backing.
If the backing consists of a porous material which has been soaked in nutrient media and dried, for example a cardboard disc which is meant to be used for differentiated bacteriological inhibition tests, a membrane filter is required adjacent to the porous backing. In this case it is not practicable to produce a composite material either by pasting the two components together, or by impregnating the porous material with the membrane filter forming component, because the layer of adhesive influences the diffusion of the nutrients between porous backing and membrane filter in an unpredictable manner and, moreover, the absence of the pure, smooth membrane surface prevents the formation of, for example, concentric inhibition circles in the test for antibiotic efficacy.